The present invention relates to a grain-oriented electrical steel sheets with magnetic properties improved by laser beam irradiation, and particularly it relates to a grain-oriented electrical steel sheets which has improved magnetic properties without laser irradiation damage generated on the steel sheet surface, as well as to a process for its production and an apparatus for realizing it.
Among conventional processes for producing a grain-oriented electrical steel sheets there have been proposed a variety of processes whereby dynamic deformation is introduced into the steel sheet surface, and a periodic closure domain is generated for fragmentation of the 180xc2x0 magnetic domain, to reduce iron loss. Among these are processes such as disclosed in Japanese Unexamined Patent Publication No. 55-18566 whereby the surface of a steel sheet is irradiated with a focused pulse YAG laser beam to introduce deformation by the evaporation counterforce of the film on the steel sheet surface, and these processes produce a grain-oriented electrical steel sheets of exceedingly high reliability and controllability because they provide a considerable improving effect on iron loss and involve non-contact working.
However, although methods which employ pulse lasers have the advantage of effectively achieving glass film evaporation counterforce on steel sheet surfaces, they leave laser irradiation damage due to breakage of the surface insulation coating. This has led to the inconvenience of requiring an insulation coating to be provided after laser irradiation.
Different methods have therefore been disclosed for minimizing damage to glass films by using continuous-wave lasers with relatively low instantaneous power, such as the technique using a continuous-wave CO2 laser described in Japanese Examined Patent Publication No. 62-49322 and the technique using a continuous-wave YAG laser described in Japanese Examined Patent Publication No. 5-32881. Particularly, in the specification relating to the latter patent it is clearly stated that since a Q-switched YAG laser has a short pulse time width and a high peak power, it is impossible to avoid evaporation and irradiation damages on glass films, so that it is not suitable for laser treatment of grain-oriented electrical steel sheets. It has also become evident that normal pulse lasers used for pulse lamp excitation and the like are unsuitable for laser treatment of grain-oriented electrical steel sheets, for the following reasons. The first reason is that, because this type of laser essentially has a very low pulse repetition rate, it cannot keep up with high-speed production lines. Another reason is that, when this type of laser is used, the average energy density on the irradiation side must be increased above that of a Q-switched pulse laser in order to achieve the necessary magnetic domain control. Increasing the average energy density on the irradiation side creates a new problem of physical deformation of the flatness of the steel sheet. Such deformation manifests itself as warping of the steel sheet and/or formation of streaks on the surface. It is stated that these streaks are detrimental to iron loss of the pulse laser-treated steel sheet, as well as detrimental to layered elements of transformers made from such pulse laser-treated steel sheets.
Incidentally, the principle of introducing deformation with a continuous-wave laser without leaving irradiation traces is based on rapid heating and rapid cooling of the steel sheet by laser irradiation. This is a major difference compared to the deformation source by the pulse laser method, which is the evaporation counterforce of the glass film.
However, while continuous-wave lasers can effectively control irradiation damages because of their low power density, their ability to achieve rapid heating and rapid cooling is lower compared to high peak power pulse lasers, resulting in lower efficiency for the introduction of deformation. Thus, in order to obtain the same improvement in iron loss through introduction of deformation as by pulse laser methods, it is necessary for the total irradiation energy on the steel sheet to be relatively higher. Incidentally, the magnetostriction of a grain-oriented steel sheet is a property which is proportional to the noise produced during its use as a transformer, and is as important a quality for grain-oriented electrical steel sheets as iron loss. In the case of laser magnetic domain control, it has been found that magnetostriction has a positive correlation with the total irradiation energy, and therefore magnetic domain control methods by continuous-wave lasers present a problem of greater magnetic deformation compared to pulse laser methods, which is a drawback of continuous-wave laser methods despite their negligible generation of irradiation damages.
In addition, when the presence of surface irradiation damages is examined closely, the phenomenon is found to be largely dependent on the irradiation power density which is determined by the beam shape and the laser power. It is therefore possible to control irradiation damages by reducing the power density. A minimum total heat input must be ensured, however, in order to produce sufficient heat deformation. With such conventional continuous-wave laser irradiation apparatuses, the heat input may be ensured by forming the laser beam as an oval with long axis in the direction of the steel sheet width, which is the scanning direction, and prolonging the time during which the laser beam is irradiated on the irradiation point. Consequently, when using irradiation apparatuses which minimize laser irradiation damages and have adjustable heat input, it has been necessary to achieve complex and precise control over the irradiation conditions, namely the laser power, scanning speed and oval beam shape.
Incidentally, the production steps for grain-oriented electrical steel sheets include annealing and insulation coating, and the steel sheet surfaces therefore comprise the oxide film formed during annealing as well as an insulation/rustproof coating applied thereover. As a result, the laser light resistance of the steel sheet surface varies minutely depending on the annealing temperature and time and on the type of coating solution. In order to minimize laser irradiation damages, therefore, it is necessary to adjust each of the laser irradiation conditions in accordance with the surface properties of the steel sheet. Among the irradiation conditions, the laser power can be controlled by the power adjusting function of the laser apparatus. The scanning speed can be easily controlled by adjusting the rotation speed of a polygon mirror or galvano mirror, which are commonly used in scanning optical systems. However, when the laser power is reduced to minimize irradiation damages, the accompanying reduction in incident heat results in insufficient introduction of deformation, and thus poorer iron loss properties. Lowering the scanning speed may therefore be considered, but this introduces the problem of a sacrifice in processing speed. Consequently, control of the laser power intensity has required control apparatuses which can be flexibly adapted not only for different laser powers and scanning speeds, but also for oval beam shapes.
In conventional irradiation apparatuses, as disclosed in the aforementioned Japanese Examined Patent Publication No. 5-32881, the laser beam focusing device is a simple cylindrical lens. With such focusing devices it is only possible to adjust oval beams in the short axis direction, and no modification can be made to the size of the beam irradiated from the laser apparatus in its long axis direction. Free and precise adjustment of oval shapes has therefore been impossible. Consequently, the prior art has been limited in minimizing laser beam damages due to minute variations in the laser light resistance of steel sheets, and this has led to practical problems in the production steps required for continuous processing of different steel sheets.
In light of this background, it has been a goal to develop a process which can produce grain-oriented electrical steel sheets with excellent magnetic properties without producing the laser irradiation damages that are a problem with pulse laser methods, and which can give improved characteristics for both iron loss and magnetostriction, as well as an apparatus for realizing such a process.
It is a first object of the present invention to provide a grain-oriented electrical steel sheet with low iron loss and very excellent magnetostriction properties.
It is a second object of the invention to provide a process for reducing iron loss of grain-oriented electrical steel sheets, which prevents surface laser irradiation damages by conventional pulse laser irradiation, which greatly minimizes increased magnetostriction which is a problem with continuous-wave lasers, and which involves laser processing steps suitable for high-speed, continuous processing.
It is a third object of the invention to provide an apparatus for producing grain-oriented electrical steel sheets with reduced iron loss of the grain-oriented electrical steel sheets by laser irradiation and with minimal surface laser irradiation damages, which is a laser irradiation apparatus easily suitable for variation in laser light resistance of a given steel sheet surface, while constantly and consistently minimizing laser irradiation damages.
The present invention relates to a grain-oriented electrical steel sheet with improved magnetic properties achieved by a reduced magnetic wall spacing with pulse laser light irradiation, which grain-oriented electrical steel sheet is characterized in that the rolling direction width of the periodic closure domain generated by laser irradiation is no greater than 150 xcexcm, the depth in the direction of the steel sheet thickness is at least 30 xcexcm, and the product of the lengths in the direction of width and the direction of depth is at least 4500 xcexcm2.
The present invention further relates to a grain-oriented electrical steel sheet with improved magnetic properties achieved by a reduced 180xc2x0 magnetic wall spacing with pulse laser light irradiation, which grain-oriented electrical steel sheet is characterized in that the rolling direction width of the periodic closure domain generated by laser irradiation is no greater than 150 xcexcm, the depth in the direction of the steel sheet thickness is at least 30 xcexcm and the product of the lengths in the direction of width and the direction of depth is at least 4500 xcexcm2, wherein the magnetostriction with materials of 0.23 mm sheet thickness (xcex19p-p compression) is no greater than 0.9xc3x9710xe2x88x926, and the magnetostriction with materials of 0.27 mm steel sheet thickness (xcex19p-p compression) is no greater than 1.3xc3x9710xe2x88x926.
The magnetostriction (xcex19p-p compression) is the stretch rate under 0.3 kg/mm2 compression stress in a 1.9 T magnetic field.
The present invention further relates to a process for producing a grain-oriented electrical steel sheet with improved magnetic properties by laser beam irradiation at equal spacing on the surface of a grain-oriented electrical steel sheet, which is a laser irradiation process whereby the laser is a pulse oscillation Q-switched CO2 laser, the irradiated beam shape is an oval with long axis in the direction of steel sheet width, the irradiation power density of the laser pulse is set to be no higher than the glass film damage threshold of the steel sheet surface to minimize laser irradiation damages, and the length of the long axis of the oval beam is set to at least the pulse beam irradiation spacing in the direction of steel sheet width to overlay a successive pulse beam on the steel sheet surface and thus provide sufficient cumulative irradiation energy necessary to improve the magnetic properties.
The present invention still further relates to an apparatus for producing a grain-oriented electrical steel sheet with improved magnetic properties by laser beam irradiation on the surface of a grain-oriented electrical steel sheet, which is an apparatus for producing a grain-oriented electrical steel sheet with excellent magnetic properties, having focusing members such as lenses or mirrors for focusing an irradiated laser beam independently provided in the steel sheet width direction and the rolling direction, having adjusting mechanisms which independently modify the distances from each focusing member to the irradiated surface of the steel sheet, and are designed to allow free adjustment of the diameter of the laser irradiation beam in the steel sheet width direction and the rolling direction. In the apparatus for producing a grain-oriented electrical steel sheet with excellent magnetic properties according to the invention, the focal length of the focusing device in the steel sheet width direction of the irradiated laser beam is adjusted to be longer than the focal length of the focusing device in the rolling direction.